The present invention relates generally to semiconductor wafer polishing, and more specifically to in-situ end point detection in semiconductor wafer polishing processes.
One common process used during the fabrication of semiconductor wafers is that of polishing the wafers. Polishing is performed for various reasons that include removing certain layers of material, exposing underlying material layers, and obtaining desired wafer thickness dimensions. Polishing processes are preferably closely monitored so that when a process end point is reached, the polishing is stopped and only the desired amount of material is actually removed from a wafer. Without such monitoring, certain material layers may be undesirably removed or left remaining on the surface of a wafer. Such process errors can ultimately degrade, or even totally prevent, operation of the resulting integrated circuit devices. Monitoring and controlling polishing processes is difficult because there are many factors, such as the condition of the polishing pad, characteristics of the slurry chemistry, the thickness of the films in the incoming wafer, and the circuit pattern density. These exemplary factors affect the time required to polish semiconductor wafers.
Semiconductor polishing is even more challenging when the physical and chemical properties of the layers in a semiconductor film structure are similar. When this is the case, it is difficult for measurement sensors to detect the processing end point at which one layer of material has been removed and next layer has been exposed. This, for example, is the case with shallow trench isolation (STI) wafers. STI wafers have multiple non-metal layers. Since it is difficult to differentiate between certain physical properties of these non-metal layers using various sensors, it is easy to either over or under polish STI wafers. Commonly, STI wafers are polished using chemical mechanical polishing techniques (CMP).
Generally, semiconductor wafer polishing control techniques can be divided into two categories. One category requires the interruption of the polishing process to remove wafers to be inspected. The other category pertains to in-situ measurement where the wafers can be inspected during the polishing process without process interruption.
A widely used method to control a semiconductor wafer polishing process is the polishing-time based method, which uses fixed polishing times determined from the polishing results of test wafers. Interruption of the polishing process is required in order to access and inspect the test wafers. Unfortunately, the interruption required to measure the test wafers requires extra processing time, thereby reducing production throughput and overall process efficiency. Also, the polishing-time based method cannot effectively handle the changing polishing conditions and the variations in the film thickness of incoming wafers, and thus often produces over or under polished results.
In-situ measurement techniques generally provide better process efficiency, however, not without its own specific performance disadvantages. Exemplary in-situ polishing measurement techniques include motor current and carrier vibration techniques. However, these techniques have disadvantages such as the inability to provide planarization information in different wafer areas and ineffectiveness for certain wafer types, such as shallow trench isolation wafers.
In view of the foregoing, improved in-situ semiconductor wafer polishing control techniques would be desirable.
The present invention relates to in-situ techniques for determining process end points in semiconductor wafer polishing processes. Generally, the technique involves utilizing a scanning inspection machine having multiple lasers and multiple detectors for detecting signals caused to emanate from an inspected specimen. The detection techniques determine the end points by differentiating between various material properties within a wafer. An accompanying algorithm is used to obtain an end point detection curve that represents a composite representation of the signals obtained from each of the detectors of the inspection machine. This end point detection curve is then used to determine the process end point. Note that computation of the algorithm is performed during the polishing process so that the process end point can be determined without interruptions that diminish process throughputs.
One aspect of the present invention pertains to a method for determining a process end point during a semiconductor wafer polishing process, using a measurement system having multiple lasers and detectors pairs that are located at different angles. The method involves repeating a monitoring cycle that includes the sequential execution of directing, measuring, recording, and determining operations. The directing operation involves directing a group of beams of radiation to be incident upon a semiconductor wafer. The reflecting light from each of the lasers is detected with a respective one of the sensors. The use of multiple sensors at different angles can effectively reduce the range of the signal intensity change during the polishing of the top oxide layer of STI wafer and thus make the end point detection at a nitride layer more reliable. The frequency of each of the measured reflectance values from multiple sensors is recorded in a two dimensional histogram. Then the average reflectance value representing the reflectance value that was most frequently measured by the sensors is determined from the histogram. The average reflectance values are fitted to a low order polynomial to form the averaged reflectance curve. The use of average reflectance values can produce a representing curve less sensitive to the circuit patterns on wafers and to other measurement noise. A non-symmetric hat function is created to provide a dynamically updated reference curve, which incorporates the properties of reflectance transitions at the interfaces between different layers of materials when polishing wafer. The process end point is identified when the average reflectance values substantially begin to deviate from the reference curve.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.